React Basics~Render Performance/ useCallback

import react, { usecallback, usestate } from "react";
import child from "./child";

const example = () => {
  console.log("parent render");

  const [counta, setcounta] = usestate(0);
  const [countb, setcountb] = usestate(0);

  const clickhandler = () => {
    setcountb((pre) => pre + 1);
  };

  return (
    <div classname="parent">
      <div>
        <h3>parent component</h3>
        <div>
          <button
            onclick={() => {
              setcounta((pre) => pre + 1);
            }}
          >
            button a
          </button>
          <span>update parent component</span>
        </div>
      </div>
      <div>
        <p>the count of click button a：{counta}</p>
      </div>
      <child countb={countb} onclick={clickhandler} />
    </div>
  );
};

export default example;

import { memo } from "react";

const childmemo = memo(({ countb, onclick }) => {
  console.log("%cchild render", "color: red;");

  return (
    <div classname="child">
      <h2>child component</h2>
      <div>
        <button onclick={onclick}>button b</button>
        <span>uodate child component</span>
      </div>
      <span>the count of click button b ：{countb}</span>
    </div>
  );
});

export default childmemo;

  const clickhandler = () => {
    setcountb((pre) => pre + 1);
  };

 <child countb={countb} onclick={clickhandler} />

  const clickHandler = useCallback(() => {
    setCountB((pre) => pre + 1);
  }, []);